The present invention relates generally to radiocommunication systems and, more particularly, to transmission during periods of speech inactivity.
Many speech communication systems apply discontinuous transmission (DTX) techniques to turn off transmitters in mobile stations (MSs) during periods of silence, e.g., pauses in conversation. This saves battery power and reduces interference caused to other users which are concurrently transmitting over the air interface.
More specifically, however, systems using DTX techniques do not completely stop transmission during periods of inactivity. In order to enable a realistic reproduction at the receiver of background noise at the transmitter, silence descriptor (SID) frames are transmitted from the transmitter to the receiver during the silent periods.
In conventional GSM Enhanced Full Rate (EFR) systems, the SID frames are synchronous to, and aligned with, the Slow Associated Control Channel (SACCH) frame structure. Such systems are described in, for example, the following standards; ETSI TC-SMG: xe2x80x9cDigital cellular telecommunications system (Phase 2+); Channel codingxe2x80x9d, GSM 05.03, version 5.2.1, EISL, November 1996, ETSI TC-SMG: Digital cellular telecommunications system; Substitution and muting of lost frames for Enhanced Full rate (EFR) speech traffic channelsxe2x80x9d, GSM 06.61, final draft version 5.1.1, ETSI, November 1996 and ETSI TC-SMG: xe2x80x9cDigital cellular telecommunications systems: Discontinuous Transmission (DTX) for Enhanced Full rate (EFR) speech traffic channelsxe2x80x9d, GSM 06.81, final draft version 5.1.1, ETSI, November 1996.
In these conventional DTX systems there is one SID frame transmitted per 24 TDMA frames (i.e., one SID frame per 480 ms). During the remainder of this period, the MS""s transmitter is turned off, unless voice activity resumes. For the SID frame transmission the same channel coding scheme is used as for a regular speech frame. Forty-three bits are used in each SID frame for comfort noise parameters which describe spectral shape and gain of the signal to be reproduced at the receiver. Ninety-five bits are used for a special SID bit pattern to identify the frame as a SID frame and to distinguish it from regular speech frames. The basic DTX principle as described in the GSM Enhanced Full Rate standard is depicted in FIG. 1. Therein, the transmission of active speech frames ends at time T1 with the transmission of a first SID frame which indicates the beginning of a period of silence and conveys the comfort noise parameters. As the period of silence continues, SID update frames are transmitted every 24 frames, e.g., at times T2 and T3. The silence periods ends at time T4 with the resumption of speech frame transmission.
There are several major drawbacks associated with these conventional discontinuous transmission systems. First, the transmitted SID frames are transmitted at relatively high power levels and produce strong interference to other users which are transmitting at the same time. Second, except for the transmitted SID frames, transmission on this channel is otherwise entirely stopped. This latter characteristic of conventional DTX systems is particularly problematic for more recent communication system innovations, such as Adaptive Multi Rate (AMR) for the GSM system. Herein it is necessary to keep track of the current channel state and to have a more or less continuous inband control channel, which both require more or less continuous reception of transmitted frames. Accordingly, it would be desirable to find a solution with the battery conservation benefits of conventional discontinuous transmission on the one hand and, on the other hand, provide continuous (or more frequent) frame transmission to satisfy the requirements of new communication system innovations. Moreover, such a solution also should eliminate the aforementioned interference bursts associated with the SID frame transmission of conventional DTX.
The foregoing drawbacks and limitations of conventional radiocommunication systems implementing DTX techniques to reduce battery drain in mobile units can be overcome according to exemplary embodiments of the present invention which provide for continuous and discontinuous transmission of SID information during periods of inactivity to allow channel monitoring and inband control data transmission to continue during those periods. At the same time, however, the SID information is transmitted at a reduced power level relative to speech and data transmissions, which is made possible by the use of more powerful error coding techniques for the SID information. Since the SID information takes fewer bits per frame to transmit than speech or data information, it is possible to use the additional bits for coding and therefore reduce the power level for transmission. Moreover, interference associated with powerful SID bursts used in conventional DTX technique is avoided.
Exemplary embodiments of the present invention transmit SID information continuously during the period of inactivity at a reduced power level. However, alternative exemplary embodiments of the present invention may transmit less than continuously, i.e., for periods of time less than or equal to the individual SID bursts used in conventional DTX techniques.